Together the foot and ankle have over 25 bones and 33 joints along with more than 100 named muscles, tendons, and ligaments and a network of blood vessels, nerves, all residing beneath a relatively slim covering of soft tissue and skin. Structurally, the foot has three main anatomical regions: the forefoot, the midfoot, and the hindfoot. These parts work together with the ankle, to provide the body with support, balance, and mobility. A structural flaw or malfunction in any one part can result in the development of problems, which are manifested in other areas of the body.
Twenty one bones make up the front part of the foot known as the forefoot, and include five metatarsal bones, fourteen phalanges that form the toes, and two sesamoid bones. The metatarsal bones and phalanges join together at the metatarsal-phalangeal (“MTP”) to form the ball of the foot. These joints play a significant role in the stability of the foot and propulsion of the lower extremity. The forefoot balances pressure on the ball of the foot and bears a substantial amount of the body weight.
The forefoot includes the five toes (which are also known as the “phalanges”) and their connecting long bones (or “metatarsals”). Several small bones together comprise a phalanx or toe. Four of the five toes have three phalanx bones respectively connected by two joints. The big toe (or “hallux”) has two phalanx bones distal and proximal with a joint in between called the interphalangeal joint. The big toe articulates with the head of the first metatarsal at the first metatarsophalangeal joint (the “MTP” joint) and there are two tiny, round bones called sesamoids on the plantar side of the metatarsal head. The phalanges are connected to the metatarsals at the ball of the foot. The two sesamoid bones are located underneath the first metatarsal bone and assist in flexion of the big toe at the first metatarsal-phalangeal joint.
The hallux is subject to several conditions which appear to present as deformities. For example, the first MTP joint can develop bunions, which is a term commonly used to refer to an outward protrusion on the medial side of the foot caused by the medial dislocation of the first and second metatarsal bones. The resulting bunion often is subjected to friction or pressure from shoes that cause swelling, redness, irritation, and pain along the side of the joint. In severe cases, the joint becomes dislocated and the phalanx of the big toe relocates and spirals diagonally (i.e., deviates laterally and/or rotates) toward the second toe.
It is believed that an instability in the mechanics of the first metatarsal (foot bone) phalangeal (toe bone) joint are the primary cause of this deformation which may be the result of one or more of the morphology of the foot, exercises to which the foot is subjected and the mechanics of footwear. Other causes are those that all may lead to a disruption in the balance of forces at the joint leading to a bunion formation and may include: congenital birth defects, neuromuscular disorders, rheumatoid arthritis, laxity in the ligaments, or trauma.
For some indications, the surgical treatment is limited to removal of a medial portion of the metatarsus to reduce the prominence of the bunion. In more severe cases, or for elderly patients, resectional arthroplasty of the metatarsophalangeal (MTP) joint, with or without implant, or joint arthrodesis may be indicated, although these procedures result in loss of motion at that joint. Fusion or “arthrodesis” of the MTP joint is a common procedure for treatment of severe bunions with the object of alleviating pain, restoring function or reducing the deformity, although most surgical procedures result in at least some loss of motion at the joint and fusion results in complete loss of motion at the joint.
Arthodesis is also performed for “hallux rigidus” which is a form of degenerative arthritis in which the proximal phalynx of the great toe is dislocated in dorsal flexion and in which movement of the joint is gradually restricted and often finally “frozen” over time. This condition can result from the inherited structure of the foot or from certain use-related trauma, such as constant balancing on the balls of the foot. Other indications that justify fusion of the joint include rheumatoid and post-traumatic arthritis and for revision of prior non-union surgeries.
The goal of surgical intervention is to restore acceptable hallux valgus, inter-metatarsal angle and distal metatarsal angles and to return the metatarsus and MTP joint to their optimum anatomic positions. Many of the current procedures involve removal of bone or an “osteotomy”. These procedures may involve removal of bone (i.e. a closing wedge osteotomy) or insertion of a wedge of bone (i.e. an opening wedge osteotomy). The success of such procedures depends on complete fixation of bone segments.
Prior art surgical methods have included fixation using bone screws, cerclage wire, K-wire, and mono-filament wire. In some cases, a plate is used to bridge the fracture or osteotomy opening or to hold the bone fragments in position during fusion.